This aim of this work is to better understand one of the important mechanisms that regulates the pattern of connections in the nervous system. The particular goal is to elucidate the way in which neurons compete to become dominant by eliminating the synapses of other neurons innervating the same target cells. New techniques (in staining and microscopy) will be used which permit visualization of nerve terminals over time in living animals. Synapses will be monitored as they undergo competitive reorganization. During this grant period, one focus will be on studies relating synaptic loss with changes in the distribution and function of postsynaptic receptor-rich areas. It is likely that changes in synaptic connections are a necessary part of the normal maturation of the nervous system and may also underly adult functions such as learning and memory. As such, understanding the biological basis of this phenomenon is fundamental to an understanding of brain maturation and function.